1. Field of the Invention
This invention relates to a rotor used for motors such as AC servo motor, brushless motor and the like, and more particularly to a rotor provided with a rare earth metal-iron type ring-shaped magnet.
2. Description of the Related Art
Heretofore, a rotor provided with a magnet has been used in motors such as AC servo motor, brushless motor and the like. In this case, the magnet is commonly used by combining a plurality of arc-shapedly divided magnet segments in a circumferential direction. As the magnet, it strongly tends to use rare earth metal-iron type magnets having good magnetic properties.
When a plurality of such arc-shapedly divided magnet segments is assembled on a rotor body to provide a rotor, the assembling efficiency of the divided magnet segments on the rotor body is good, but the formation of magnetic pole is dependent upon the shape of the magnet divided into arc or the like.
For this end, it is attemped to optionally conduct the setting of the magnetic pole by using a ring-shaped rare earth metal-iron type magnet without dividing into arc or the like.
However, when such a ring-shaped rare earth metal-iron type magnet is assembled on the rotor body, the thermal expansion coefficient of the ring-shaped rare earth metal-iron type magnet is small, while the thermal expansion coefficient of the rotor body (e.g. made from S45C, SUS403, Al or the like) is large, so that if the temperature of the rotor rises, the thermal expansion quantity of the rotor body becomes larger than that of the ring-shaped rare earth metal-iron type magnet, and hence the cracking or breakage is undesirably caused in the ring-shaped rare earth metal-iron type magnet.
In order to solve this problem, it is attempted to form a large gap between the rotor body and the ring-shaped rare earth metal-iron type magnet or to insert an adhesive into such a large gap. In this case, however, the concentric accuracy between the rotor body and the ring-shaped rare earth metal-iron type magnet may lower due to the formation of the large gap or the degradation of the adhesive, so that these attempts are not effective at the present time.
Further, in order to prevent the scattering of broken ring-shaped rare earth metal-iron type magnet or to improve the corrosion resistance of the rare earth metal-iron type magnet having a poor corrosion resistance in the motor requiring the high reliability, a coating for the prevention of the scattering and the improvement of the corrosion resistance may be arranged around the outer peripheral surface of the ring-shaped magnet, but when the thermal expansion coefficient of the coating is large, if the rotor is placed in a low-temperature environment, the thermal shrinkage quantity of the coating becomes larger than that of the ring-shaped rare earth metal-iron type magnet, and hence the cracking or breakage is caused in the ring-shaped rare earth metal-iron type magnet.